The present invention relates generally to a projection exposure method and a projection exposure apparatus. In particular, the present invention relates to a projection exposure method for illuminating a mask having a pattern formed thereon with exposure light so as to expose an image of the pattern formed on the mask onto a photosensitive substrate through a projection optical system, and a projection exposure apparatus to which this exposure method is applied. The present invention is characterized in that a focusing operation is conducted at the center of an exposure region on the photosensitive substrate, irrespective of the shape of the exposure region.
A projection exposure apparatus for transferring a mask pattern onto a photosensitive substrate mounted on a stage by using exposure light such as ultraviolet light through a projection optical system has been put to practical use in various precision machining fields, for example, the manufacture semiconductor integrated circuits. This projection exposure apparatus requires a focusing system for setting the exposure shot region on the photosensitive substrate within a width corresponding to the focal depth of the imaging plane of the projection optical system, or a so-called automatic focusing system.
Such a focusing system is typically classified into (1) a direct system or (2) an indirect system. In system (1), i.e., the direct system, the focusing position of the image of the mask pattern in a reference plane provided on a stage is directly detected by using the exposure light. More specifically, as disclosed in, for example, Japanese Laid-Open Patent Publication No. 1-286418, a special mark formed on the surface of the mask pattern is projected onto the reference plane. The projected image of the mark formed on the reference plane is observed through the projection optical system and the mark so that the light quantity peak of the projected image regulated by the mark is detected, thereby discriminating the focusing position.
In system (2), i.e., the indirect system, a measuring section for measuring the height of the stage relative to the projection optical system is separately provided. The origin of the measuring section is made coincident with the focusing position, which is previously obtained using the aforementioned direct system, and the height of the exposure surface of the photosensitive substrate is detected by means of the measuring section, so that the exposure surface is guided to the focusing position indirectly. As an example of such a measuring section for measuring the stage height, for example, Japanese Laid-Open Patent Publication No. 1-41962 corresponding to U.S. Pat. No. 4,650,983 discloses a system for measuring the height of the exposure surface just below the projection optical system by using an obliquely incidence optical system fixed onto the outer side of the projection optical system.
Also, as an example, a system in which a special mark formed on the mask pattern surface is projected directly onto the exposure surface of the photosensitive substrate, and the projected image is detected through the projection optical system and the mark, thereby the focusing is directly discriminated, is disclosed in, for example, Japanese Laid-Open Patent Publication No. 57-212406.
Under such surroundings, in a case of semiconductor memory devices for which particularly high processing accuracy is required, a projection optical system with a focal depth of about 1 .mu.m or less that uses i-ray of 365 nm in wavelength has been recently used. In this case, positioning accuracy of the focusing position of 0.1 .mu.m or less is usually required, whereas an extremely high accuracy of 0.05 .mu.m or less is required in another projection exposure method utilizing an interference phenomenon of the exposure light such as the one disclosed in, for example, Japanese Patent Publication 62-50811.
In recent years, an exposure process in which a plurality of chips are exposed at a time, for example, a four-chips-per-shot exposure process or a six-chips-per-shot exposure process is performed, as the exposed area is increased. In such cases, there are instances where it is desired to expose only a part of an exposable region of the projection optical system. Also, there are other instances where it is necessary to form a pattern for tests (a TEG pattern) only on a part of a wafer. In addition, there are other instances where it is desired to expose different wafers having different chip-sizes by means of the same projection exposure apparatus or where it is desired to expose a plurality kind of patterns onto different shot regions of a single photosensitive substrate, respectively, by using a reticle on which the patterns are formed. In these cases, the exposure region must be varied by performing a masking process. Therefore, a variable field stop is provided within the illuminating optical system so as to vary the shape of the exposure region.
In the conventional projection exposure apparatus, however, the focusing system (the automatic focusing system) has the detection center (detection location) only at a determined location in the exposable region (which is usually at the center of an exposable region of the projection system, i.e., on the optical axis of the projection optical system). Therefore, even in the case where only a part of the exposable region of the projection optical system is to be exposed by varying the shape of the blind for the reasons mentioned above, the focusing operation is performed at the center of the projection optical system (the projection lens). This results in exposure while the exposure region determined by the blind is not precisely in focus, thereby degrading resolution.